Geologic Hazards
Several natural hazards are the result of the geologic conditions beneath the surface of the Midwest. Three hazards that are common in the Midwest are earthquakes, sinkholes, and radon. Although the Midwest does not lie on a plate boundary, the New Madrid fault is currently active and capable of generating earthquakes. Sinkholes are caused by the dissolving of limestone and dolostone rocks beneath the surface of some regions in the Midwest. Radon is a gas that is released from the natural breakdown of radioactive elements found in bedrock. Although these three hazards are not unique to the Midwest, they do pose some challenges for people living in this area.
Earthquakes
While earthquakes usually bring California to mind, the New Madrid and Wabash seismic zones are responsible for earthquakes throughout the southern Midwest (Figure 10.5). The New Madrid seismic zone is in the Mississippi Valley, at the boundaries of Arkansas, Tennessee, Kentucky, Illinois, and Missouri. The Wabash Valley seismic zone extends northward along the boundary between Illinois and Indiana. These seismic zones are poorly understood because, unlike other seismic zones, there is nothing on the surface to help scientists understand the faults responsible for the seismic activity. For example, there is a thick layer of river-deposited sediments (called alluvium) that covers what is thought to be a strike slip fault. Microseismic earthquakes that are too small to be felt by humans happen every other day, but larger earthquakes are fairly rare. The bedrock that makes up most of the central US is colder, drier, and less fractured than rocks on the East or West Coast. As a result, the earthquakes here can release the same amount of energy as other earthquakes, but the shaking affects a much larger area because the seismic waves travel through denser, more solid bedrock.
Figure 10.5: Earthquake events in the New Madrid and Wabash Valley seismic zones.
Earthquake prediction is very difficult because most of the mechanisms that cause earthquakes are beneath the Earth’s surface. Scientists typically make use of historical records as well as limited surface monitoring to understand the probability of a seismic event occurring. Historical reports of earthquakes from 1811 to 1812 indicate a two-month period that included several major earthquakes thought to be greater than 7.0 in magnitude. Based on these historical reports, and the absence of a large magnitude earthquake in the past century, scientists expect that the New Madrid seismic zone is overdue for a large magnitude earthquake.
Figure 10.6: Karst topography in the continental US.
A new research study is likely to better explain the seismic activity of the Midwest. EarthScope is a large-scale project funded by the National Science Foundation to gather data about the lithosphere beneath the Earth’s surface. There are three large investigations of the seismic activity of this area. A network of 400 temporary seismometers has been installed, with a distance of approximately 70 kilometers (40 miles) between each seismometer. The volume of data collected by these seismometers will help geophysicists create three-dimensional models of the lithosphere. With these models, we will have a virtual picture of the seismic zones, and, hopefully, we will come to better understand the seismic hazards of the Midwest.
Sinkholes
Sinkholes are usually caused by a geologic feature known as karst topography. Karst can form where the underlying bedrock is composed of material that can be slowly dissolved by water. Much of the Midwest has carbonate bedrock consisting of limestone, dolostone, and marble. These particular types of sedimentary rock contain significant amounts of carbonate (carbon atoms combined with multiple oxygen atoms). Water that mixes with carbon dioxide in the air and soil reacts to produce carbonic acid (H2CO3). This acid and the carbonate react, dissolving the rock. Although this takes a long time to occur, much of the bedrock in the Midwest is Paleozoic in age (between 541 million years and 252 million years old) (Figure 10.6). Eventually, caves and caverns form in the rock, and sinkholes form when caves near the surface collapse. The karst topography is noticeable in areas that contain many sinkholes and where the land surface is scattered with large, round depressions.
There are a few conditions that increase the hazards associated with sinkholes. The previously mentioned underground caves are often filled with groundwater. In regions where there is a rapidly growing population, a greater amount of water is extracted from the ground. Likewise, during periods of drought, the water table can drop considerably, leaving these caves filled with air instead of the water that would normally help to support the weight of the ground above. Without the support of the ground water, the surface can collapse. In some cases, increasing the weight on the surface by rapidly building large structures can also lead to sinkhole collapse. There are a few steps that can be taken to mitigate these issues. Geologists and environmental engineers can use ground-penetrating radar to identify the location of sinkholes. In some areas where sinkholes are very prevalent, engineers can fill the sinkholes with gravel, which will allow for water drainage while still supporting the surface.
Radon
Radon is a naturally occurring radioactive, colorless, odorless gas. It is the leading cause of lung cancer in non-smokers and the second leading cause of lung cancer overall. It can collect in homes, buildings, and even in the water supply. The bedrock geology and glacial history of the Midwest provides ideal conditions for radon hazards.
Radon is one of the products of decay from the breakdown of radioactive elements in soil, rock, and water. Uranium-238 undergoes radioactive decay, producing energy and several radioactive products, such as Radon-222 and Thorium-232, the latter of which decays to emit energy and Radon-220. Uranium and Thorium are naturally occurring radioactive elements found in bedrock such as granite, shale, and limestone. Most of the Midwest has bedrock consisting of shale, sandstone, limestone, and dolostone, all of which can contain radon (Figure 10.7). Most of the northern Midwest has been substantially eroded from the glaciers of the Pleistocene. These glaciers left behind large deposits of gravel, sand, and clay as they receded and melted. The gravel, sand, and clay are often the eroded remnants of radon-containing bedrock, and the moist temperate climate of the Midwest provides an excellent environment for weathering of both the bedrock and the glacial sediments. As they weather, more rock is exposed, which, in turn, allows more radon to be released.
Figure 10.7: Radon zone map of the US. (Note: Zone 1 contains the highest radon levels.)
Radon gas finds its way through cracks in the basement foundation, sump pump wells, dirt floor crawlspaces in the basement, and basement floor drains. Radon can be found in water from wells and municipal water. Since radon is more easily released from warm water than from cold water, one of the greatest forms of exposure likely occurs while showering in water with high radon levels. Fortunately, with proper monitoring and mitigation (reduction) techniques, radon gas can be easily reduced to low levels. One technique that is often used in homes involves sealing cracks in the basement floor, covering drains, and installing ventilation systems. A well ventilated space will prevent the radon from accumulating and will reduce the risk of exposure. Most states have licensed radon mitigation specialists who are trained in the proper testing and mitigation of radon levels in buildings. The EPA has published a homebuyer’s guide designed to help citizens make informed decisions about radon gas. For radon in water, filtration systems can be installed to mitigate exposure in the home.
